diff --git a/types/heartbeat.go b/types/heartbeat.go
index 40a7b01b0..64676ea64 100644
--- a/types/heartbeat.go
+++ b/types/heartbeat.go
@@ -10,8 +10,11 @@ import (
 	cmn "github.com/tendermint/tmlibs/common"
 )
 
-// Heartbeat is a simple vote-like structure so validators can alert others that
-// they are alive and waiting for transactions.
+// Heartbeat is a simple vote-like structure so validators can
+// alert others that they are alive and waiting for transactions.
+// Note: We aren't adding ",omitempty" to Heartbeat's
+// json field tags because we always want the JSON
+// representation to be in its canonical form.
 type Heartbeat struct {
 	ValidatorAddress data.Bytes       `json:"validator_address"`
 	ValidatorIndex   int              `json:"validator_index"`
@@ -22,6 +25,7 @@ type Heartbeat struct {
 }
 
 // WriteSignBytes writes the Heartbeat for signing.
+// It panics if the Heartbeat is nil.
 func (heartbeat *Heartbeat) WriteSignBytes(chainID string, w io.Writer, n *int, err *error) {
 	wire.WriteJSON(CanonicalJSONOnceHeartbeat{
 		chainID,
@@ -31,6 +35,9 @@ func (heartbeat *Heartbeat) WriteSignBytes(chainID string, w io.Writer, n *int,
 
 // Copy makes a copy of the Heartbeat.
 func (heartbeat *Heartbeat) Copy() *Heartbeat {
+	if heartbeat == nil {
+		return nil
+	}
 	heartbeatCopy := *heartbeat
 	return &heartbeatCopy
 }
diff --git a/types/heartbeat_test.go b/types/heartbeat_test.go
new file mode 100644
index 000000000..8a0967128
--- /dev/null
+++ b/types/heartbeat_test.go
@@ -0,0 +1,56 @@
+package types
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/tendermint/go-crypto"
+)
+
+func TestHeartbeatCopy(t *testing.T) {
+	hb := &Heartbeat{ValidatorIndex: 1, Height: 10, Round: 1}
+	hbCopy := hb.Copy()
+	require.Equal(t, hbCopy, hb, "heartbeat copy should be the same")
+	hbCopy.Round = hb.Round + 10
+	require.NotEqual(t, hbCopy, hb, "heartbeat copy mutation should not change original")
+
+	var nilHb *Heartbeat
+	nilHbCopy := nilHb.Copy()
+	require.Nil(t, nilHbCopy, "copy of nil should also return nil")
+}
+
+func TestHeartbeatString(t *testing.T) {
+	var nilHb *Heartbeat
+	require.Contains(t, nilHb.String(), "nil", "expecting a string and no panic")
+
+	hb := &Heartbeat{ValidatorIndex: 1, Height: 11, Round: 2}
+	require.Equal(t, hb.String(), "Heartbeat{1:000000000000 11/02 (0) {<nil>}}")
+
+	var key crypto.PrivKeyEd25519
+	hb.Signature = key.Sign([]byte("Tendermint"))
+	require.Equal(t, hb.String(), "Heartbeat{1:000000000000 11/02 (0) {/FF41E371B9BF.../}}")
+}
+
+func TestHeartbeatWriteSignBytes(t *testing.T) {
+	var n int
+	var err error
+	buf := new(bytes.Buffer)
+
+	hb := &Heartbeat{ValidatorIndex: 1, Height: 10, Round: 1}
+	hb.WriteSignBytes("0xdeadbeef", buf, &n, &err)
+	require.Equal(t, string(buf.Bytes()), `{"chain_id":"0xdeadbeef","heartbeat":{"height":10,"round":1,"sequence":0,"validator_address":"","validator_index":1}}`)
+
+	buf.Reset()
+	plainHb := &Heartbeat{}
+	plainHb.WriteSignBytes("0xdeadbeef", buf, &n, &err)
+	require.Equal(t, string(buf.Bytes()), `{"chain_id":"0xdeadbeef","heartbeat":{"height":0,"round":0,"sequence":0,"validator_address":"","validator_index":0}}`)
+
+	require.Panics(t, func() {
+		buf.Reset()
+		var nilHb *Heartbeat
+		nilHb.WriteSignBytes("0xdeadbeef", buf, &n, &err)
+		require.Equal(t, string(buf.Bytes()), "null")
+	})
+}